Batrachotoxinin-A-20-alpha-p-3H-benzoate (3H-BTX-B) is a potent analog of batrachotoxin which exhibits a selective action on sodium channels in a variety of excitable tissues resulting in a decrease of the resting membrane potential. 3H-BTX-B binds to a specific and saturable receptor site in homogenized mouse cortex preparations with a dissociation constant of 7 times 10 to the minus 7th power M. Our study proposes to characterize this binding with respect to temperature, pH, ion concentrations, stoichiometry, and interactions with other agents and toxins known to affect the sodium channel or the action of batrachotoxin. Binding studies under different conditions will be carried out by a pellet assay wherein specific binding is measured as the difference between total binding in the presence and absence of a 100-fold excess of unlabeled toxin. These studies will be extended to other excitable tissues. The results will be analyzed and, taken in conjunction with existent data on the electrophysiological effects of the toxin, should lead to a better understanding of the sodium channel and provide a foundation for subsequent use of BTX as a molecular tool in the study of excitability mechanisms.